To enhance the contrast of the interference pattern and hence to allow a more accurate and faster analysis, it is attractive when the reference beam and the measuring beam have an opposite polarization. From the thesis “Development of a wafer geometry measuring system” by M. Jansen, Eindhoven, 2006, an interferometer is known, comprising a light source, adapted to generate a coherent light beam, a detector adapted to analyze the phase difference of optical light beams, location means for locating an object to be measured, a first optical path from the light source to the object and a second optical path from the object to the detector, wherein the first and the second optical path have a common section adjacent to the object.
This prior art interferometer requires the use of polarizing and non-polarizing beam splitters and a quarter wave plate. These are complicated, bulky and expensive parts, which are difficult to adjust. Further the newly proposed concept can use a coherent light source and the obtained interference fringes will have a better contrast. Compared to FIG. 4.26c of this prior art document, the advantage in the newly proposed concept is found in the fact that the reference plane does not require to be tilted relative to the tested object.
Just like any other polarization encoded dynamic interferometer, the newly proposed interferometer also requires a phase analyzer which may consists of either the principle of FIG. 4.24a of this prior art document, using polarizing and non polarizing cube beam splitters, FIG. 4.24b of this prior art document or a phase analyzer as described in patent US-A-2005/0046864. All of these designs require a quarter waveplate at 45° relative to the polarization axis of the reference beam, to be positioned in front of the phase analyser. The waveplate is required to convert the orthogonally polarized reference and test beam into two counterrotating circularly polarized beams such that the beams can be made to interfere by passing it through a analysing polarizer as described in FIG. 4.22 of the thesis. A phase shift can then be introduced by changing the orientation of the analysing polarizer.
The document US-A-2006/0146340 could also be used as a prior art document as it discloses roughly the same contents as the thesis in relation to the present invention.